Why does my pet rat not like being picked up?

I have 3 rats which I've had for about a year an a half and one of them has never really let me pick her up much. She squeaks whenever I try to, but I'm pretty sure I'm not hurting her. Once they're all out and running around she's fine and she'll crawl all over me. I don't think she's got anything wrong with her because I've had pat rets since I was about 5. It would just be nice to be able to just pick her up out of the cage without it sounding like she's in pain.